It is well-known to use an immersion nozzle for pouring molten steel from a tundish into a mold in the continuous casting of steel. In the course of the continuous casting, alumina (Al.sub.2 O.sub.3) contained in the liquid steel or produced by the oxidation of Aluminum (Al) contained in the liquid steel, can adhere to the inner surface of the nozzle and cause what is commonly called "nozzle blockage" in which the nozzle is blocked by alumina inclusions.
Thus, in order to prevent this nozzle blockage and thereby perform smooth casting, inert gas such as Argon (Ar) gas, Nitrogen (N.sub.2) gas or a similar inert gas is injected from an upper nozzle or a lower nozzle, which is connected to the immersion nozzle, such that the alumina inclusions on the inner wall of the nozzle hole can be removed. This method is presently applied to an upper or lower nozzle which is connected to an immersion nozzle, an air seal pipe or a similar device as shown in FIGS. 4 to 6.
FIG. 4 shows an example of the mounting construction in which an immersion nozzle is connected to a tundish. As shown in this figure, an upper nozzle 10 is inserted into the bottom of a tundish located over this construction. Slide plates 11, 12 are placed on the underside of the upper nozzle. Generally, the upper slide plate 11 is fixed and the lower slide plate 12 is slidably attached so as to open/close a lower nozzle 20 connected to the lower slide plate 12. An immersion nozzle 30 is connected to the lower nozzle 20.
FIG. 5 shows an example of the mounting construction of an air seal pipe 40, which is used for pouring liquid steel from a ladle into a tundish. The ladle is provided over this structure and the tundish is located below it.
An upper nozzle 10 is mounted on the bottom of the ladle. Slide plates 11, 12 are placed on the downside of this upper nozzle 10 in order to regulate the flow rate of liquid steel being poured into the tundish. Generally, the upper slide plate 11 is fixed and the lower slide plate 12 is slidably attached so as to regulate the flow rate of molten steel. A lower nozzle 20 is provided below the lower slide plate 12 and an air seal pipe 40 is mounted on the underside of the lower nozzle 20.
FIG. 6 shows an example of the mounting construction in which an open nozzle 50 is mounted on the bottom of a tundish. Slide plates 11, 12 are attached to the underside of an upper nozzle 10. The open nozzle 50 is placed on the underside of the lower slide plate 12.
In each of the above-mentioned examples, inert gas is introduced from a gas injection pipe 5 into the upper nozzle 10 in order to prevent the blockage of this nozzle. The blockage of the lower nozzle is also prevented in the same manner as shown in the upper nozzle 10.
FIG. 7 is a detailed sectional view of an upper nozzle 10 of the prior art. This gas injection nozzle is composed of a permeable porous refractory 4 and has a nozzle hole 3 through which liquid metal passes. Generally, the whole circumference of this nozzle is surrounded by an iron shell 2 and is formed such that the gas introduced through a gas injection pipe 5 can be blown into the nozzle hole 3.
Generally, a gas pool 1a, which is resistant to heat, is provided in the middle portion of the porous refractory 4. This gas pool 1a is formed as a space between a portion of the circumference of the porous refractory 4 and the corresponding portion of an iron shell 2. Due to the existence of this gas pool 1a, the gas supplied through the gas supplying pipe can penetrate into the porous refractory 4 and leak into the nozzle hole 3, which makes it possible to prevent such inclusions as alumina from depositing on the inner surface of the nozzle hole 3.
As shown in FIG. 7., the nozzle hole 3 is formed as a cylindrical space between the porous refractory 4 and the iron shell 2. When liquid metal has passed through the nozzle hole 3, the temperature of the nozzle hole 3 is high at its upper portion and low at its lower portion. Thus, thermal stress is produced in the axial direction of the porous refractory 4. As shown in FIG. 7, this thermal stress can lead to a crack 6 which occurs in a direction perpendicular to the axial direction of the porous refractory 4 in the upper side of the gas pool 1a.
FIG. 8 is a detailed sectional view of a lower nozzle 20 of the prior art. Similarly, as in the case of the upper nozzle mentioned above, the thermal stress produced in the axial direction of the porous refractory 4 can lead to a crack 6, which occurs in a direction perpendicular to this axial direction.